Multiple doctor blade coating apparatus

ABSTRACT

Providing a final doctor blade in a multiple blade coating apparatus having a surface dimension, in the direction of coated web movement, of at least five times that of the proceeding doctor blades.

United States Patent PATENTEU JUN 1 5 Isn MULTIPLE DOCTOR BLADE COATINGAPPARATUS This application is a continuation of application Ser. NoY

746,176, filed July 19, 1968 and novil abandoned.

The present invention relates generally to a multiple doctor bladecoating apparatus and more particularly to an improved apparatussuitable for applying successively and at a high speed, a plurality ofphotographic fluid coating compositions such as gelatino silver halideemulsions, an emulsion for protective layer, an emulsion forantihalation layer, an emulsion for a filter layer and an emulsion for amasking layer, in applying photographic light sensitive materials to thesurface of a continuously travelling web such as a film base or barytapaper, in one step, and without setting or drying the coated layersbetween their application.

We have previously proposed a multiple doctor blade coating apparatus inU.S. Pat. application Ser. No. 589,896 in which a coating devicecomprising chambers containing fluid coating compositions (a singlechamber for each composition). Discharge openings supply each coatingcomposition to the surface of the travelling web separately, each of thedischarge openings being connected to a corresponding chamber and facingthe web. Doctor blades partition each discharge opening and meter eachcoating composition onto the travelling web, which is spaced slightlyfrom the surface of the travelling web. Each coating composition issupplied to the corresponding chamber by means of a suitable supplyer ata rate corresponding to a desired thickness of coating, and each appliesits coating composition separately, through an associated dischargeopening to the surface of the travelling web without forming spacesbetween the coating compositions. The coating compositions aresuperimposed on the travelling web and metered by a corresponding doctorblade, whereby distinct, superimposed, multiple layers of the coatingcompositions are formed on the travelling web.

ln applying a coating composition to the surface ofa web, it isgenerally desired to reduce the thickness of the coated layer forfacilitating drying of the coated layer, to increase the coating speed,and also to reduce the size of the drying equipment to reduce theoverall cost of drying.

ln the aforesaid multiple doctor blade coating apparatus, the thicknessof each coated layer may be considerably reduced as compared to aconventional single layer doctor blade coating system. However, in orderto further reduce the coating thickness using a conventional multipledoctor blade coating apparatus, the clearance between the coating deviceand the web to be coated must be further reduced. However, it isundesirable to reduce the clearance between the coating device and theweb over definite limits since it is accompanied by the followingdefects: cometlilte spots are caused by dust or other foreign solidmaterials deposited on or embedded in the back surface of the web or thesurface ofa backup roller; coating defects are caused by the unevennessin thickness of web or local deformation of the web, and further thethickness of the coated layer becomes uneven due to poor finishing ofthe surface of the backup roller or the top of the doctor blade.Moreover, in the case when ajoined portion of the web passes through thespace existing between the coating device or the backup roller, it isnecessary to move the coating device or the backup roller to momentarilyenlarge the clearance therebetween.

Therefore, an object of the present invention is to provide an improvedmultiple doctor blade coating apparatus for achieving thin layer coatingwithout narrowing the clearance between a coating device and atravelling web to be coated.

Another object of this invention is to provide a multiple doctor coatingapparatus for applying, in one step, a plurality of coating compositionsin thin multiple layers at a high speed.

Still another object of this invention is to provide a doctor coatingapparatus for uniformly applying to a continuously travelling web,distinct, superimposed, multiple, thin layers of a plurality of coatingcompositions.

A further object of this invention is to provide a multiple doctorcoating apparatus suitable for simply and economically preparingphotographic light sensitive materials involving multiple photographiclayers of photographic coating compositions.

We have noted that the length of each doctor blade in the travellingdirection of the web is an important factor for determining theclearance between the coating device and the web in multiple doctorcoating and have found that in the case where each coating compositionhas the same viscosity, a good coating can be achieved by adjusting theclearance between the coating device'and the web to the value determinedbythe following equation:

E L3 (A) where:

'FLL-mj j=1,2,-,n C: the clearance 4between a coating device and a web(micron) Ly: the length (in the travelling direction of web) of thedoctor blade corresponding to j-th coating composition (mm.)

t,: the thickness of i-th coated layer (micron) n: the number of coatingcompositions to be coated on the web simultaneously.

According to the present invention, a plurality of coating compositionscan be applied to a web in thin layers without reducing the clearancebetween a coating device and the web by suitably selecting the length ofeach doctor blade in the travelling direction of web based on theaforesaid equation, that is, by making the length of the doctor bladefor the coating composition of the uppermost layer in the travellingdirection of web longer than the length of each of other doctor blades.

The present invention will be explained in detail by referring to theaccompanying drawings; in which:

FIG. 1 is a cross-sectional elevational view of an embodiment of thecoating apparatus of the present invention, and

FIG. 2 is a fragmentary, enlarged., cross-sectional view of the coatingsection of the doctor blade coating apparatus shown in FIG. l.

Now referring to FIG. 1 and FIG. 2, a coating device 2 is positionedslightly apart from a continuously travelling web l to be coated whilesupported by a backup roller. The coating apparatus comprises a seriesof separate chambers 3, 4, 5, and 6, each containing coatingcompositions for lst, 2nd, 3rd, and 4th layers respectively and eachconnected to means (not shown) for supplying separate coatingcompositions (not shown) thereto. Discharging openings or slots 7, 8, 9,and 10 are provided for applying to, web 1 each coating compositionseparately, with the slots each connected to corresponding chambersmentioned above. Doctor blades 15, 16, 17 and 18 partition the aforesaiddischarging slots from each other and meter each coating composition tothe web from the discharging slots. The doctor blades each have smoothtop surfaces, finished with high accuracy facing the web. Doctorchannels 11, 12, 13 and 14 are formed between the doctor blades and theweb. A back plate 19 together with doctor blade 15 forms the dischargeslot for the lst coating composition.

Each coating composition is supplied to the corresponding chamber fromsupply means (not shown) at a rate corresponding to the desiredthickness of coating. The compositions are applied directly to acontinuously travelling web l, separately from each discharge slot oropening to fill the doctor channel formed between the doctor blades andthe web and without forming spaces between the coating compositions.Distinct superimposed layers of the coating compositions aresuccessively formed while metering the coating compositions by thecorresponding doctor blades respectively in a desired ratio ofthickness.

ln the apparatus of this invention, the length L4 of the top plane ofthe doctor blade 18 in the travelling direction of web l andcorresponding to the coating composition for the uppermost layer is from5 to 20 mm., and must be longer than five times the length L1, L2, andLa ofthe top planes of the doctor blades 1S, I6, and 17 respectively inthe travelling direction of the web. The length Ll, L2, and La ofthe topsurfaces of the doctor blades l5, 16, and 17 in the travelling directionof web are preferably from 0.5 to 2 mm. Furthermore, ifa further thinthickness of the first layer (closest to the web surface) of coatingcomposition is desired, the length L, of the doctor blade l5, in thetravelling direction ofweb and corresponding to the first layer ofcoating composition is desirably shorter than 0.5 mm.

The advantages obtained by using the multiple doctor blade coatingapparatus of this invention will be sufficiently understood by thefollowing practical calculations conducted by use of equation (A) asmentioned above.

CALCULATION 1 Number of coatings, 4 Viscosity of each coatingcomposition, 30 cp. Thickness of each coated layer, 50 microns Multipledoctor coating apparatus Previous ofthe doctor present coating inventionapparatus Length of doctor blade:

1, mm. 1 1 L2, )Ilm 1 1 L3, mm 1 1 L4, mm l C, microns.. 350 250 'Theclearance between a coating device and a web.

ln applying four coating compositions to a web having the same thicknessfor the four layers, the clearance between the coating device and theweb, in the multiple doctor coating apparatus of this invention, may be40 percent greater than that of employing a conventional doctor coatingapparatus as mentioned above. In other words, as will be easilyunderstood from the above calculation, the thickness of each coatedlayer can be effectively reduced by employing the multiple doctor bladecoating apparatus of this invention.

CALC ULATION 2 Number ot coatings, 4 Viscosity of each coatingcomposition, 30 cp. Clearance between a coating device and a Web, 250microns Multiple doctor coating Previous apparatus multiple of thedoctor present coating invention apparatus Length of doctor blade:

L1, IDID c 1 1 1 Lz, mm.. 1 1 L3, mm.. 1 1 L4, mm 10 1 Sum o! thicknessof coated layers, microns. 141 200 3Q cp.

170 microns.

'The clearance between a coating device and a web.

By using the multiple doctor coating apparatus of the present invention,the clearance between the coating device and the web can be increased byabout 60 percent over that of employing a conventional multiple doctorblade coating apparatus when the total thickness of the coated layers iskept constant.

As is clear from the above-mentioned calculations, by using the multipledoctor blade coating apparatus of this invention, the clearance betweena coating device and a web can be effectively reduced when a constantthickness of each coated layer is desired or the thickness of eachcoated layer can be effectively reduced when the clearance between thecoating device and the web is constant. Therefore, according to thepresent invention it becomes possible to practice high speed thin layermultiple doctor blade coating. This has been impossible to practice inthe past, owing to the formation of cometlike spots caused by dust orother foreign solid matter deposited on or embedded in the back surfaceof a travelling web or the surface ofa backup roller; the formation ofcoating defects caused by the unevenness in thickness of web or apartial deformation of web; the unevenness in thickness of coatingthickness caused by the poor accuracy in finish of the doctor blades orthe backup roller; and the difficulty in passing the joined portion ofweb through the clearance or channel between the coating device and theweb. i

In addition, while the aforesaid calculations were made only for thecase where each coating composition has the same viscosity, the presentinvention can be effectively applied to cases where coating compositionshaving different viscosity are employed.

Also, besides these advantages, the multiple doctor coating apparatus ofthe present invention has, as a matter of course, the advantages commonto conventional multiple doctor coating apparatus as mentioned above.Since a plurality of fluid coating compositions can be applied to atravelling web in one step without setting or drying each coatingcomposition separately, they are all coated during one coatingoperation. Efficiency is extremely improved in the case where aphotographic light sensitive material having multiple layers such as acolor photographic material is prepared by means of the multiple coatingapparatus of the present invention.

Since the coating compositions continuously contact each other in thespace or channel between the coating device and the web, the wettingproperty between the coating compositions is better, which facilitatesthe selection of surface active agents and neither stripes nor commets"which frequently form in the case of conventional multiple doctor bladecoating apparatus are formed and uniform coating is achieved. Since thecoating compositions contact a continuously travelling web in the spaceexisting between the coating device and the web, the wetting propertybetween the web and the coating compositions is improved, the airentrapped by the travelling web does not enter the space between thecoating compositions and the web, even when the speed ofthe travellingweb is increased. Also, the resistance ofthe coating compositions in thespace between the coating device and the web to ambient disturbance isincreased, which makes it possible to coat at high speed.

Since the coating compositions are successively superimposed on atravelling web while being metered by a corresponding doctor blade.there sc'arcely occurs mixing and other disturbances of coatingcompositions at the interface between the coating compositions and henceuniform and distinct superimposed multiple layers of coatingcompositions can be formed.

EXAMPLE By using the doctor blade coating apparatus illustrated in FIG.l, four coating compositions for a cinecolor positive film were appliedto an undercoated cellulose triacetate film in one step under thefollowing conditions:

(')z blue-sensitive emulsion; (")z red-sensitive emulsion; ("izgreen-sensitive emulsion; aqueous gelatin solution for protective layer.

The clearance between the coating device and the web was about 350microns, which coincides with the calculation result obtained by theequation (A) shown before, and a good coating was achieved.

On the other hand, when the coating was conducted under the aforesaidconditions using the conventional multiple doctor blade coatingapparatus wherein L1, L2, L3, and L4 were l mm., good coating wasimpossible where the clearance between the coating device and the webwas greater than 250 microns.

ln addition, when the color photographic film was tested no cometlikespots were formed, the thickness of the coated layers was uniform, nointermixing of coating compositions between them was observed, and thephotographic properties were better,

lt is intended that the invention shall not be limited to the kinds ofcoating compositions and shape of the coating device as shown above.

What I claim is:

1. A multiple doctor coating apparatus for successively applying aplurality of coating compositions to the surface of a web travellingover a backup surface comprising:

a. a coating-head having a plurality of chambers each adapted to receivea coating composition from a suitable supply means;

b. each of said chambers having a discharge opening, each of saiddischarge openings being successively adjacent each other in thedirection of travel of said web with a doctor blade forming a downstreamportion of each discharge opening in the direction of travel of said weband located in close proximity to said web;

c. said doctor blades having each an end surface facing said websubstantially parallel thereto and spaced substan tially equally distanttherefrom, said doctor blades acting to meter each of said coatingcompositions as the coating compositions are discharged from theirrespective opening into the space between said doctor blades and thesurface of said web;

d. each of said end surfaces of said doctor blades having a substantialwidth in the direction of travel of said web with the width of the endsurface of the doctor blade metering the topmost layer being at least 5times greater than the width of the end surfaces of the other doctorblades, each of the end surfaces of said other doctor blades beingapproximately 2 mm.

